Modified Nanobentonites for Water Remediation

ESTEFANIA BRACCO; MATÍAS BUTLER; ROBERTO CANDAL; PATRICIO CARNELLI; LUCAS GUZ; FEDERICO IVANIC; ELSA LOPEZ LOVEIRA; JOSÉ LUIS MARCO BROWN

Industrial Applications of Nanoparticles A Prospective Overview

CRC Press

Año: 2023;

(sheet silicate minerals formed by parallel layers of silicate tetrahedra) composed of alternating tetrahedral and octahedral layers in a 2:1 ratio. MMT is a nanoclay with a layer thickness around 1 nm (Barakan and Aghazadeh 2021); the lattice has an excess negative charge, which is balanced in the interlayer space by hydrated inorganic cations, such as Na(I), Ca(II), etc. Therefore, MMT has a high Cation Exchange Capacity (CEC) and a large surface area, leading to high adsorption capacities for pollutants (qAwad et al. 2019).Bentonites can be—and have been—used to treat a wide variety of target pollutants, from complex organic molecules, including herbicides, fungicides and pharmaceutical drugs, to a broad range of metals with different speciation (Pandey 2017, Prabhu and Prabhu 2018, Awad et al. 2019, Yadav et al. 2019). Its extensive applicability, in addition to its low cost and easy accessibility, makes bentonite a useful alternative compared with more aggressive remediation methods. Its adsorption capacity on different types of target pollutants arises from various and accessible modifications that can be built into its structure.The most common modification of the bentonite structure is the intercalation of cations (Fe, Al, etc.) into its layers, as this increases the interlayer spacing and therefore, the adsorption capacity, usually accentuating the hydrophilic nature of the clay by the presence of hydrated cations (Calabi Floody et al. 2009). The adsorption behavior of iron exchanged nanobentonite clay and Fe3O4 nanoparticles for removing NO3− and HCO3− from wastewater was compared, and a higher efficiency of the modified bentonite was observed, as a result of its higher specific surface area and increased basal spacing (Mukhopadhyay et al. 2019).Despite bentonite having an overall neutral charge, the excess negative charge on its lattice may provide affinity to cationic species and to a lesser extent, to anionic and neutral molecules. Thus, organic modifiers are usually included in its structure for improving affinity to organic compounds. L-tryptophan (Trp) was added to the structure of a modified bentonite in order to benefit fromthe amphoteric nature of amino acids and to improve the selectivity and adsorption capacity of the bentonite for organic molecules (Gallouze et al. 2021). Comparing the achieved results amongnon-modified bentonite, Na-bentonite, Fe-Na-bentonite and Trp-Na-bentonite, the latter exhibited the greatest adsorption capacity for the emerging contaminant 17α-ethinylestradiol (EE2), that led to the conclusion that it may be a promising low-cost adsorbent resource, not only for EE2 and steroids but also for other organic pollutants.In another example of removal of organic pollutants, bentonite was modified by two cationic surfactants (octadecyltrimethylammonium -C18- and dioctadecyltrimethylammonium -2C18-) to remove triclosan, an antibacterial agent, toxic to humans and other living organisms (Phuekphong et al. 2020). These modifications allowed a higher surface hydrophobicity and a greater adsorption capacity of the mentioned contaminant, especially when using 2C18 modified bentonite. Furthermore, the reusability of the latter was tested, finding only a slight decrease in the adsorption capacity after washes. Improved adsorption methods may not only be cheaper but also more ecofriendly than other removal techniques.Chemical modifications of clays such as acid treatment, intercalation of organic compounds and pillaring by different metal polyoxycations may be combined to expand the sorption capacity and selectivity of bentonites. A montmorillonite modified with a cationic surfactant (cetyltrimethylammonium, CTMA), followed by an acid treatment and subsequent introduction of iron hydroxides was synthesized, the material showing the ability to adsorb simultaneously oxyanions, hydrophobic compounds and heavy metals in batch experiments in water (Yang et al. 2020).Moreover, the applicability of modified and unmodified bentonites can be further expanded through coupling with other methods, thus enhancing the removal of pollutants. For example, nanoscale zerovalent iron (nZVI) has been recently used for remediation purposes, especially as a permeable reactive barrier, due to the reducing ability of Fe0. The performance of nZVI particles attached over montmorillonite (MMT-nZVI) and monodispersed nZVI particles in removing Cr (VI), a well-studied carcinogenic element, was studied (Yin et al. 2020). It was concluded that MMT-nZVI presented a similar removal yield of Cr (VI) from water compared with monodispersed elemental Fe nanoparticles, but much higher than aggregated nZVI particles and MMT alone, as this material provides a higher porous area and prevents particle aggregation.At the same time, the removal of contaminants and target molecules promoted by the adsorption affinity of bentonites can be followed by a complete or partial degradation using Advanced Oxidation Processes (AOPs). AOPs rely on the formation of various oxidant radicals (primarily the hydroxyl radical, HO•), which may react with the contaminant and transform it into more oxidized products. A classic example of AOPs are the Fenton and photo-Fenton processes, which consist of a pH-dependent array of reactions employing a catalytic amount of Fe (II) to react with H2O2 and form Fe (III) and HO•; the photo-Fenton process uses UV or solar light to regenerate the Fe (II) more efficiently (Langford and Carey 1975, Kim and Vogelpohl 1998).Other examples are in the simultaneous use of clay minerals and photocatalysis. The presence of clay seemed to affect the reaction yields and product selectivity in the oxidation of benzene (Ide et al. 2012) and 2-chlorophenol (Mogyorósi et al. 2002) in water using TiO2 photocatalysts and UV radiation. A few other examples include: an organically modified bentonite combined with TiO2 for the removal of toluene (Chen et al. 2011) and a smectite-TiO2 film used for the degradation of methyl orange and methylene blue (Deepracha et al. 2019).A common way of exploiting the combination of AOPs and adsorption techniques is the removal and storage of the pollutants using the adsorbent material followed by desorption and treatment of the sample. An advantage of bentonites, as mentioned earlier, is the wide variety of modifications that can be built into their structures, thereby facilitating the coupling and enhancing the yield of transformation. This means that the same material can be utilized to adsorb and transform the target pollutant. A modified bentonite synthesized through a pillaring process using Fe and Al cations was used to treat water samples containing an anionic dye (Congo red, CR) (Khelifi and Ayari 2019). These alterations not only enhanced the adsorbent capacity towards the dye but also made possible its use as a catalyst by the addition of UV light and H2O2, allowing mineralization of the adsorbed pollutant molecules and the recovery of the adsorbent through a photo-Fenton process, with a degradation percentage of 98%.Bentonites can also be converted into catalysts for AOPs by the incorporation of nanoparticles of iron oxides. A modified organo-bentonite was prepared with CTMA, which later favored the fixation of TiO2 and Fe2O3, showing promising heterogeneous photo-Fenton activity for the removal of two pharmaceutical drugs in water (Molina et al. 2020). The degradation of an organic pollutant, diethylphthalate ester (DEP) by photo-Fenton-like reactions was compared using free Fe2O3 nanoparticles and Fe2O3 nanoparticles embedded in montmorillonite clay particles (Fe-MMT). Experiments were performed at pH 5.5 in the presence of citric acid as an iron complexing agent. The Fe-MMT system exhibited a higher photocatalytic efficiency for DEP degradation than the pure Fe2O3 nanoparticles due to its porous structure, large surface area and more oxygen vacancies. Additionally, the agglomeration of Fe2O3 nanoparticles is also prevented when they are supported on MMT. Moreover, the recovery and regeneration of the material were improved, achieving a degradation efficiency of more than 50% after three recycling cycles (Sun et al. 2021).There are also examples of novel uses that involve composites of bentonites with other adsorbents, such as: the combination of an organoclay and activated carbon to treat water containing benzene, toluene, xylene, naphthalene and various oils (Alther 2002), arsenic and lead (Mangwandi et al. 2016, Mo et al. 2018), in batch mode, and organic micropollutants such as carbamazepine, 4-tert-octylphenol, 4-nonylphenol and anthracene in a continuous mode (Kamińska 2018). A Montmorillonite-Graphene oxide Composite (MGC) was tested to treat water containing Pb2+ and p-nitrophenol (PNP), although the removal of the separated pollutants (97% for PNP and 99% Pb2+) appeared to be more efficient than when combined (98% Pb2+ and 51% for PNP) due to Pb2+ strong competitiveness (Zhang et al. 2019).Although membrane filtration is an established method for water remediation, filtration efficiency and avoiding fouling still need improving. Organically-modified clays can be used either in membranes made of polymer-clay nanocomposites (Alshangiti et al. 2019) or in the combination of adsorption on clay minerals with membrane filtration (Shaalan 2009).Finally, the combination of bacteria, archaea and fungi with bentonites could be a sustainable and environmentally friendly bioremediation alternative. The adsorption properties of bentonites provide protection to microbes from toxic substances, thus enhancing their biodegradation capabilities and the mycorrhizal symbiotic associations of fungi with plants, which is of great importance for phytoremediation (Gadd 2010, Dong 2012). There are other environmental applications for the combination of bentonites with microorganisms and enzymes. For example, enzymes can be immobilized onto a bentonite support that provides protective properties allowing their use as biosensors or biocatalysts (An et al. 2015).This chapter presents different strategies for the modification of bentonites to increase their versatility and capacity as adsorbents for pollutants and describes different mechanisms proposed for the adsorption of organic compounds. Some applications as catalysts for Fenton andphoto-Fenton-like processes are also discussed.